From 6c73ca4a2f4ea71f4a880670624e7b2fdb6f32da Mon Sep 17 00:00:00 2001 From: Clemens Lang Date: Fri, 18 Nov 2022 12:35:33 +0100 Subject: signature: Clamp PSS salt len to MD len MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit FIPS 186-4 section 5 "The RSA Digital Signature Algorithm", subsection 5.5 "PKCS #1" says: "For RSASSA-PSS […] the length (in bytes) of the salt (sLen) shall satisfy 0 <= sLen <= hLen, where hLen is the length of the hash function output block (in bytes)." Introduce a new option RSA_PSS_SALTLEN_AUTO_DIGEST_MAX and make it the default. The new value will behave like RSA_PSS_SALTLEN_AUTO, but will not use more than the digest length when signing, so that FIPS 186-4 is not violated. This value has two advantages when compared with RSA_PSS_SALTLEN_DIGEST: (1) It will continue to do auto-detection when verifying signatures for maximum compatibility, where RSA_PSS_SALTLEN_DIGEST would fail for other digest sizes. (2) It will work for combinations where the maximum salt length is smaller than the digest size, which typically happens with large digest sizes (e.g., SHA-512) and small RSA keys. J.-S. Coron shows in "Optimal Security Proofs for PSS and Other Signature Schemes. Advances in Cryptology – Eurocrypt 2002, volume 2332 of Lecture Notes in Computer Science, pp. 272 – 287. Springer Verlag, 2002." that longer salts than the output size of modern hash functions do not increase security: "For example,for an application in which at most one billion signatures will be generated, k0 = 30 bits of random salt are actually sufficient to guarantee the same level of security as RSA, and taking a larger salt does not increase the security level." Signed-off-by: Clemens Lang Reviewed-by: Dmitry Belyavskiy Reviewed-by: Tomas Mraz (Merged from https://github.com/openssl/openssl/pull/19724) --- doc/man7/EVP_SIGNATURE-RSA.pod | 5 +++++ 1 file changed, 5 insertions(+) (limited to 'doc/man7') diff --git a/doc/man7/EVP_SIGNATURE-RSA.pod b/doc/man7/EVP_SIGNATURE-RSA.pod index 440e1c634f..de6869786c 100644 --- a/doc/man7/EVP_SIGNATURE-RSA.pod +++ b/doc/man7/EVP_SIGNATURE-RSA.pod @@ -68,6 +68,11 @@ Use the maximum salt length. Auto detect the salt length. +=item "auto-digestmax" (B) + +Auto detect the salt length when verifying. Maximize the salt length up to the +digest size when signing to comply with FIPS 186-4 section 5.5. + =back =back -- cgit v1.2.3