Internet Engineering Task Force (IETF) J. Schaad Request for Comments: 6955 Soaring Hawk Consulting Obsoletes: 2875 H. Prafullchandra Category: Standards Track HyTrust, Inc. ISSN: 2070-1721 May 2013
Diffie-Hellman Proof-of-Possession Algorithms
Abstract
This document describes two methods for producing an integrity check value from a Diffie-Hellman key pair and one method for producing an integrity check value from an Elliptic Curve key pair. This behavior is needed for such operations as creating the signature of a Public- Key Cryptography Standards (PKCS) #10 Certification Request. These algorithms are designed to provide a Proof-of-Possession of the private key and not to be a general purpose signing algorithm.
This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc6955.
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Copyright Notice
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Table of Contents
1. Introduction ....................................................3 1.1. Changes since RFC 2875 .....................................4 1.2. Requirements Terminology ...................................5 2. Terminology .....................................................5 3. Notation ........................................................5 4. Static DH Proof-of-Possession Process ...........................6 4.1. ASN.1 Encoding .............................................8 5. Discrete Logarithm Signature ...................................11 5.1. Expanding the Digest Value ................................11 5.2. Signature Computation Algorithm ...........................12 5.3. Signature Verification Algorithm ..........................13 5.4. ASN.1 Encoding ............................................14 6. Static ECDH Proof-of-Possession Process ........................16 6.1. ASN.1 Encoding ............................................18 7. Security Considerations ........................................20 8. References .....................................................21 8.1. Normative References ......................................21 8.2. Informative References ....................................21 Appendix A. ASN.1 Modules .........................................23 A.1. 2008 ASN.1 Module ..........................................23 A.2. 1988 ASN.1 Module ..........................................28 Appendix B. Example of Static DH Proof-of-Possession ..............30 Appendix C. Example of Discrete Log Signature .....................38
Among the responsibilities of a Certification Authority (CA) in issuing certificates is a requirement that it verifies the identity for the entity to which it is issuing a certificate and that the private key for the public key to be placed in the certificate is in the possession of that entity. The process of validating that the private key is held by the requester of the certificate is called Proof-of-Possession (POP). Further details on why POP is important can be found in Appendix C of RFC 4211 [CRMF].
This document is designed to deal with the problem of how to support POP for encryption-only keys. PKCS #10 [RFC2986] and the Certificate Request Message Format (CRMF) [CRMF] both define syntaxes for Certification Requests. However, while CRMF supports an alternative method to support POP for encryption-only keys, PKCS #10 does not. PKCS #10 assumes that the public key being requested for certification corresponds to an algorithm that is capable of producing a POP by a signature operation. Diffie-Hellman (DH) and Elliptic Curve Diffie-Hellman (ECDH) are key agreement algorithms and, as such, cannot be directly used for signing or encryption.
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This document describes a set of three POP algorithms. Two methods use the key agreement process (one for DH and one for ECDH) to provide a shared secret as the basis of an integrity check value. For these methods, the value is constructed for a specific recipient/ verifier by using a public key of that verifier. The third method uses a modified signature algorithm (for DH). This method allows for arbitrary verifiers.
It should be noted that we did not create an algorithm that parallels the Elliptical Curve Digital Signature Algorithm (ECDSA) as was done for the Digital Signature Algorithm (DSA). When using ECDH, the common practice is to use one of a set of predefined curves; each of these curves has been designed to be paired with one of the commonly used hash algorithms. This differs in practice from the DH case where the common practice is to generate a set of group parameters, either on a single machine or for a given community, that are aligned to encryption algorithms rather than hash algorithms. The implication is that, if a key has the ability to perform the modified DSA algorithm for ECDSA, it should be able to use the correct hash algorithm and perform the regular ECDSA signature algorithm with the correctly sized hash.
o The Static DH POP algorithm has been rewritten for parameterization of the hash algorithm and the Message Authentication Code (MAC) algorithm.
o New instances of the Static DH POP algorithm have been created using the Hashed Message Authentication Code (HMAC) paired with the SHA-224, SHA-256, SHA-384, and SHA-512 hash algorithms. However, the current SHA-1 algorithm remains identical.
o The Discrete Logarithm Signature algorithm has been rewritten for parameterization of the hash algorithm.
o New instances of the Discrete Logarithm Signature have been created for the SHA-224, SHA-256, SHA-384, and SHA-512 hash functions. However, the current SHA-1 algorithm remains identical.
o A new Static ECDH POP algorithm has been added.
o New instances of the Static ECDH POP algorithm have been created using HMAC paired with the SHA-224, SHA-256, SHA-384, and SHA-512 hash functions.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].
When the words are in lower case they have their natural language meaning.
The following definitions will be used in this document:
DH certificate = a certificate whose SubjectPublicKey is a DH public value and is signed with any signature algorithm (e.g., RSA or DSA).
ECDH certificate = a certificate whose SubjectPublicKey is an ECDH public value and is signed with any signature algorithm (e.g., RSA or ECDSA).
Proof-of-Possession (POP) = a means that provides a method for a second party to perform an algorithm to establish with some degree of assurance that the first party does possess and has the ability to use a private key. The reasoning behind doing POP can be found in Appendix C in [CRMF].
This section describes mathematical notations, conventions, and symbols used throughout this document.
a | b : Concatenation of a and b a ^ b : a raised to the power of b a mod b : a modulo b a / b : a divided by b using integer division a * b : a times b Depending on context, multiplication may be within an EC or normal multiplication
KDF(a) : Key Derivation Function producing a value from a MAC(a, b) : Message Authentication Code function where a is the key and b is the text LEFTMOST(a, b) : Return the b left most bits of a FLOOR(a) : Return n where n is the largest integer such that n <= a
The Static DH POP algorithm is set up to use a Key Derivation Function (KDF) and a MAC. This algorithm requires that a common set of group parameters be used by both the creator and verifier of the POP value.
The steps for creating a DH POP are:
1. An entity (E) chooses the group parameters for a DH key agreement.
This is done simply by selecting the group parameters from a certificate for the recipient of the POP process. A certificate with the correct group parameters has to be available.
Let the common DH parameters be g and p; and let the DH key pair from the certificate be known as the recipient (R) key pair (Rpub and Rpriv).
Rpub = g^x mod p (where x=Rpriv, the private DH value)
2. The entity generates a DH public/private key pair using the group parameters from step 1.
For an entity (E):
Epriv = DH private value = y Epub = DH public value = g^y mod p
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3. The POP computation process will then consist of the following steps:
(a) The value to be signed (text) is obtained. (For a PKCS #10 object, the value is the DER-encoded certificationRequestInfo field represented as an octet string.)
(b) A shared DH secret is computed as follows:
shared secret = ZZ = g^(x*y) mod p
[This is done by E as Rpub^y and by the recipient as Epub^x, where Rpub is retrieved from the recipient's DH certificate (or is provided in the protocol) and Epub is retrieved from the Certification Request.]
(c) A temporary key K is derived from the shared secret ZZ as follows:
LeadingInfo ::= Subject Distinguished Name from recipient's certificate
TrailingInfo ::= Issuer Distinguished Name from recipient's certificate
(d) Using the defined MAC function, compute MAC(K, text).
The POP verification process requires the recipient to carry out steps (a) through (d) and then simply compare the result of step (d) with what it received as the signature component. If they match, then the following can be concluded:
(a) The entity possesses the private key corresponding to the public key in the Certification Request because it needs the private key to calculate the shared secret; and
(b) Only the recipient that the entity sent the request to could actually verify the request because it would require its own private key to compute the same shared secret. In the case where the recipient is a CA, this protects the entity from rogue CAs.
The algorithm outlined above allows for the use of an arbitrary hash function in computing the temporary key and the MAC algorithm. In this specification, we define object identifiers for the SHA-1, SHA-224, SHA-256, SHA-384, and SHA-512 hash values and use HMAC for the MAC algorithm. The ASN.1 structures associated with the Static DH POP algorithm are:
In the above ASN.1, the following items are defined:
DhSigStatic This ASN.1 type structure holds the information describing the signature. The structure has the following fields:
issuerAndSerial This field contains the issuer name and serial number of the certificate from which the public key was obtained. The issuerAndSerial field is omitted if the public key did not come from a certificate.
hashValue This field contains the result of the MAC operation in step 3(d) (Section 4).
sa-dhPop-static-sha1-hmac-sha1 An ASN.1 SIGNATURE-ALGORITHM object that associates together the information describing a signature algorithm. The structure DhSigStatic represents the signature value, and the parameters MUST be absent.
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id-dhPop-static-sha1-hmac-sha1 This OID identifies the Static DH POP algorithm that uses SHA-1 as the KDF and HMAC-SHA1 as the MAC function. The new OID was created for naming consistency with the other OIDs defined here. The value of the OID is the same value as id-dh-sig-hmac-sha1, which was defined in the previous version of this document [RFC2875].
sa-dhPop-static-sha224-hmac-sha224 An ASN.1 SIGNATURE-ALGORITHM object that associates together the information describing this signature algorithm. The structure DhSigStatic represents the signature value, and the parameters MUST be absent.
id-dhPop-static-sha224-hmac-sha224 This OID identifies the Static DH POP algorithm that uses SHA-224 as the KDF and HMAC-SHA224 as the MAC function.
sa-dhPop-static-sha256-hmac-sha256 An ASN.1 SIGNATURE-ALGORITHM object that associates together the information describing this signature algorithm. The structure DhSigStatic represents the signature value, and the parameters MUST be absent.
id-dhPop-static-sha256-hmac-sha256 This OID identifies the Static DH POP algorithm that uses SHA-256 as the KDF and HMAC-SHA256 as the MAC function.
sa-dhPop-static-sha384-hmac-sha384 An ASN.1 SIGNATURE-ALGORITHM object that associates together the information describing this signature algorithm. The structure DhSigStatic represents the signature value, and the parameters MUST be absent.
id-dhPop-static-sha384-hmac-sha384 This OID identifies the Static DH POP algorithm that uses SHA-384 as the KDF and HMAC-SHA384 as the MAC function.
sa-dhPop-static-sha512-hmac-sha512 An ASN.1 SIGNATURE-ALGORITHM object that associates together the information describing this signature algorithm. The structure DhSigStatic represents the signature value, and the parameters MUST be absent.
id-dhPop-static-sha512-hmac-sha512 This OID identifies the Static DH POP algorithm that uses SHA-512 as the KDF and HMAC-SHA512 as the MAC function.
When a single set of parameters is used for a large group of keys, the chance that a collision will occur in the set of keys, either by accident or design, increases as the number of keys used increases. A large number of keys from a single parameter set also encourages the use of brute force methods of attack, as the entire set of keys in the parameters can be attacked in a single operation rather than having to attack each key parameter set individually.
For this reason, we need to create a POP for DH keys that does not require the use of a common set of parameters.
This POP algorithm is based on DSA, but we have removed the restrictions dealing with the hash and key sizes imposed by the [FIPS-186-3] standard. The use of this method does impose some additional restrictions on the set of keys that may be used; however, if the key-generation algorithm documented in [RFC2631] is used, the required restrictions are met. The additional restrictions are the requirement for the existence of a q parameter. Adding the q parameter is generally accepted as a good practice, as it allows for checking of small subgroup attacks.
The following definitions are used in the rest of this section:
p is a large prime g = h^((p-1)/q) mod p, where h is any integer 1 < h < p-1 such that h^((p-1)/q) mod p > 1 (g has order q mod p) q is a large prime j is a large integer such that p = q*j + 1 x is a randomly or pseudo-randomly generated integer with 1 < x < q y = g^x mod p HASH is a hash function such that b = the output size of HASH in bits
Note: These definitions match the ones in [RFC2631].
Besides the addition of a q parameter, [FIPS-186-3] also imposes size restrictions on the parameters. The length of q must be 160 bits (matching the output length of the SHA-1 digest algorithm), and the length of p must be 1024 bits. The size restriction on p is eliminated in this document, but the size restriction on q is replaced with the requirement that q must be at least b bits in length. (If the hash function is SHA-1, then b=160 bits and the size restriction on b is identical with that in [FIPS-186-3].) Given that
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there is not a random length-hashing algorithm, a hash value of the message will need to be derived such that the hash is in the range from 0 to q-1. If the length of q is greater than b, then a method must be provided to expand the hash.
The method for expanding the digest value used in this section does not provide any additional security beyond the b bits provided by the hash algorithm. For this reason, the hash algorithm should be the largest size possible to match q. The value being signed is increased mainly to enhance the difficulty of reversing the signature process.
This algorithm produces m, the value to be signed.
Let L = the size of q (i.e., 2^L <= q < 2^(L+1)). Let M be the original message to be signed. Let b be the length of HASH output.
1. Compute d = HASH(M), the digest of the original message.
2. If L == b, then m = d.
3. If L > b, then follow steps (a) through (d) below.
(a) Set n = FLOOR(L / b)
(b) Set m = d, the initial computed digest value
(c) For i = 0 to n - 1 m = m | HASH(m)
(d) m = LEFTMOST(m, L-1)
Thus, the final result of the process meets the criteria that 0 <= m < q.
The signature verification process is far more complicated than is normal for DSA, as some assumptions about the validity of parameters cannot be taken for granted.
Given a value m to be validated, the signature value pair (r, s) and the parameters for the key:
1. Perform a strong verification that p is a prime number.
2. Perform a strong verification that q is a prime number.
3. Verify that q is a factor of p-1; if any of the above checks fail, then the signature cannot be verified and must be considered a failure.
In the above ASN.1, the following items are defined:
sa-dhPop-sha1 A SIGNATURE-ALGORITHM object that associates together the information describing this signature algorithm. The structure DSA-Sig-Value represents the signature value, and the structure DomainParameters SHOULD be omitted in the signature but MUST be present in the associated key request.
id-alg-dhPop-sha1 This OID identifies the Discrete Logarithm Signature using SHA-1 as the hash algorithm. The new OID was created for naming consistency with the others defined here. The value of the OID is the same as id-alg-dh-pop, which was defined in the previous version of this document [RFC2875].
sa-dhPop-sha224 A SIGNATURE-ALGORITHM object that associates together the information describing this signature algorithm. The structure DSA-Sig-Value represents the signature value, and the structure DomainParameters SHOULD be omitted in the signature but MUST be present in the associated key request.
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id-alg-dhPop-sha224 This OID identifies the Discrete Logarithm Signature using SHA-224 as the hash algorithm.
sa-dhPop-sha256 A SIGNATURE-ALGORITHM object that associates together the information describing this signature algorithm. The structure DSA-Sig-Value represents the signature value, and the structure DomainParameters SHOULD be omitted in the signature but MUST be present in the associated key request.
id-alg-dhPop-sha256 This OID identifies the Discrete Logarithm Signature using SHA-256 as the hash algorithm.
sa-dhPop-sha384 A SIGNATURE-ALGORITHM object that associates together the information describing this signature algorithm. The structure DSA-Sig-Value represents the signature value, and the structure DomainParameters SHOULD be omitted in the signature but MUST be present in the associated key request.
id-alg-dhPop-sha384 This OID identifies the Discrete Logarithm Signature using SHA-384 as the hash algorithm.
sa-dhPop-sha512 A SIGNATURE-ALGORITHM object that associates together the information describing this signature algorithm. The structure DSA-Sig-Value represents the signature value, and the structure DomainParameters SHOULD be omitted in the signature but MUST be present in the associated key request.
id-alg-dhPop-sha512 This OID identifies the Discrete Logarithm Signature using SHA-512 as the hash algorithm.
6. Static ECDH Proof-of-Possession Process
The Static ECDH POP algorithm is set up to use a KDF and a MAC. This algorithm requires that a common set of group parameters be used by both the creator and the verifier of the POP value. Full details of how Elliptic Curve Cryptography (ECC) works can be found in RFC 6090 [RFC6090].
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The steps for creating an ECDH POP are:
1. An entity (E) chooses the group parameters for an ECDH key agreement.
This is done simply by selecting the group parameters from a certificate for the recipient of the POP process. A certificate with the correct group parameters has to be available.
The ECDH parameters can be identified either by a named group or by a set of curve parameters. Section 2.3.5 of RFC 3279 [RFC3279] documents how the parameters are encoded for PKIX certificates. For PKIX-based applications, the parameters will almost always be defined by a named group. Designate G as the group from the ECDH parameters. Let the ECDH key pair associated with the certificate be known as the recipient key pair (Rpub and Rpriv).
Rpub = Rpriv * G
2. The entity generates an ECDH public/private key pair using the parameters from step 1.
For an entity (E):
Epriv = entity private value Epub = ECDH public point = Epriv * G
3. The POP computation process will then consist of the following steps:
(a) The value to be signed (text) is obtained. (For a PKCS #10 object, the value is the DER-encoded certificationRequestInfo field represented as an octet string.)
shared secret value ZZ is the x coordinate of the computed point
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(c) A temporary key K is derived from the shared secret ZZ as follows:
K = KDF(LeadingInfo | ZZ | TrailingInfo)
LeadingInfo ::= Subject Distinguished Name from certificate TrailingInfo ::= Issuer Distinguished Name from certificate
(d) Compute MAC(K, text).
The POP verification process requires the recipient to carry out steps (a) through (d) and then simply compare the result of step (d) with what it received as the signature component. If they match, then the following can be concluded:
(a) The entity possesses the private key corresponding to the public key in the Certification Request because it needed the private key to calculate the shared secret; and
(b) Only the recipient that the entity sent the request to could actually verify the request because it would require its own private key to compute the same shared secret. In the case where the recipient is a CA, this protects the entity from rogue CAs.
The algorithm outlined above allows for the use of an arbitrary hash function in computing the temporary key and the MAC value. In this specification, we define object identifiers for the SHA-1, SHA-224, SHA-256, SHA-384, and SHA-512 hash values. The ASN.1 structures associated with the Static ECDH POP algorithm are:
sa-ecdhPop-sha512-hmac-sha512 SIGNATURE-ALGORITHM ::= { IDENTIFIER id-alg-ecdhPop-static-sha512-hmac-sha512 VALUE DhSigStatic PARAMS ARE absent PUBLIC-KEYS { pk-ec } }
These items reuse the DhSigStatic structure defined in Section 4. When used with these algorithms, the value to be placed in the field hashValue is that computed in step 3(d) (Section 6). In the above ASN.1, the following items are defined:
sa-ecdhPop-static-sha224-hmac-sha224 An ASN.1 SIGNATURE-ALGORITHM object that associates together the information describing this signature algorithm. The structure DhSigStatic represents the signature value, and the parameters MUST be absent.
id-ecdhPop-static-sha224-hmac-sha224 This OID identifies the Static ECDH POP algorithm that uses SHA-224 as the KDF and HMAC-SHA224 as the MAC function.
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sa-ecdhPop-static-sha256-hmac-sha256 An ASN.1 SIGNATURE-ALGORITHM object that associates together the information describing this signature algorithm. The structure DhSigStatic represents the signature value, and the parameters MUST be absent.
id-ecdhPop-static-sha256-hmac-sha256 This OID identifies the Static ECDH POP algorithm that uses SHA-256 as the KDF and HMAC-SHA256 as the MAC function.
sa-ecdhPop-static-sha384-hmac-sha384 An ASN.1 SIGNATURE-ALGORITHM object that associates together the information describing this signature algorithm. The structure DhSigStatic represents the signature value, and the parameters MUST be absent.
id-ecdhPop-static-sha384-hmac-sha384 This OID identifies the Static ECDH POP algorithm that uses SHA-384 as the KDF and HMAC-SHA384 as the MAC function.
sa-ecdhPop-static-sha512-hmac-sha512 An ASN.1 SIGNATURE-ALGORITHM object that associates together the information describing this signature algorithm. The structure DhSigStatic represents the signature value, and the parameters MUST be absent.
id-ecdhPop-static-sha512-hmac-sha512 This OID identifies the Static ECDH POP algorithm that uses SHA-512 as the KDF and HMAC-SHA512 as the MAC function.
7. Security Considerations
None of the algorithms defined in this document are meant for use in general purpose situations. These algorithms are designed and purposed solely for use in doing POP with PKCS #10 and CRMF constructs.
In the Static DH POP and Static ECDH POP algorithms, an appropriate value can be produced by either party. Thus, these algorithms only provide integrity and not origination service. The Discrete Logarithm Signature algorithm provides both integrity checking and origination checking.
All the security in this system is provided by the secrecy of the private keying material. If either sender or recipient private keys are disclosed, all messages sent or received using those keys are compromised. Similarly, the loss of a private key results in an inability to read messages sent using that key.
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Selection of parameters can be of paramount importance. In the selection of parameters, one must take into account the community/ group of entities that one wishes to be able to communicate with. In choosing a set of parameters, one must also be sure to avoid small groups. [FIPS-186-3] Appendixes A and B.2 contain information on the selection of parameters for DH. Section 10 of [RFC6090] contains information on the selection of parameters for ECC. The practices outlined in these documents will lead to better selection of parameters.
[CRMF] Schaad, J., "Internet X.509 Public Key Infrastructure Certificate Request Message Format (CRMF)", RFC 4211, September 2005.
[FIPS-186-3] National Institute of Standards and Technology, "Digital Signature Standard (DSS)", Federal Information Processing Standards Publication 186-3, June 2009, <http://www.nist.gov/>.
[RFC2875] Prafullchandra, H. and J. Schaad, "Diffie-Hellman Proof-of-Possession Algorithms", RFC 2875, July 2000.
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[RFC3279] Bassham, L., Polk, W., and R. Housley, "Algorithms and Identifiers for the Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile", RFC 3279, April 2002.
[RFC5912] Hoffman, P. and J. Schaad, "New ASN.1 Modules for the Public Key Infrastructure Using X.509 (PKIX)", RFC 5912, June 2010.
[RFC6090] McGrew, D., Igoe, K., and M. Salter, "Fundamental Elliptic Curve Cryptography Algorithms", RFC 6090, February 2011.
This appendix contains an ASN.1 module that is conformant with the 2008 version of ASN.1. This module references the object classes defined by [RFC5912] to more completely describe all of the associations between the elements defined in this document. Where a difference exists between the module in this section and the 1988 module, the 2008 module is the definitive module.
BEGIN -- EXPORTS ALL -- The types and values defined in this module are exported for use -- in the other ASN.1 modules. Other applications may use them -- for their own purposes.
This appendix contains an ASN.1 module that is conformant with the 1988 version of ASN.1, which represents an informational version of the ASN.1 module for this document. Where a difference exists between the module in this section and the 2008 module, the 2008 module is the definitive module.
BEGIN -- EXPORTS ALL -- The types and values defined in this module are exported for use -- in the other ASN.1 modules. Other applications may use them -- for their own purposes.
Appendix B. Example of Static DH Proof-of-Possession
The following example follows the steps described earlier in Section 4.
Step 1. Establishing common DH parameters: Assume the parameters are as in the DER-encoded certificate. The certificate contains a DH public key signed by a CA with a DSA signing key.
Step 3. The hash value needs to be expanded, since |q| = 256. This is done by hashing the hash with SHA1 and appending it to the original hash. The value after this step is:
Next, the first 255 bits of this value are taken to be the resulting "hash" value. Note that in this case a shift of one bit right is done, since the result is to be treated as an integer: