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RSA_SIGN(3) Library Functions Manual RSA_SIGN(3)

RSA_sign, RSA_verify
RSA signatures

#include <openssl/rsa.h>
int
RSA_sign(int type, const unsigned char *m, unsigned int m_len, unsigned char *sigret, unsigned int *siglen, RSA *rsa);
int
RSA_verify(int type, const unsigned char *m, unsigned int m_len, unsigned char *sigbuf, unsigned int siglen, RSA *rsa);

RSA_sign() signs the message digest m of size m_len using the private key rsa using RSASSA-PKCS1-v1_5 as specified in RFC 3447. It stores the signature in sigret and the signature size in siglen. sigret must point to RSA_size(rsa) bytes of memory. Note that PKCS #1 adds meta-data, placing limits on the size of the key that can be used. See RSA_private_encrypt(3) for lower-level operations.
type denotes the message digest algorithm that was used to generate m. If type is NID_md5_sha1, an SSL signature (MD5 and SHA1 message digests with PKCS #1 padding and no algorithm identifier) is created.
RSA_verify() verifies that the signature sigbuf of size siglen matches a given message digest m of size m_len. type denotes the message digest algorithm that was used to generate the signature. rsa is the signer's public key.

RSA_sign() returns 1 on success. RSA_verify() returns 1 on successful verification.
The error codes can be obtained by ERR_get_error(3).

ERR_get_error(3), RSA_new(3), RSA_private_encrypt(3), RSA_public_decrypt(3)

SSL, PKCS #1 v2.0

RSA_sign() first appeared in SSLeay 0.4.4. RSA_verify() first appeared in SSLeay 0.6.0. Both functions have been available since OpenBSD 2.4.
March 27, 2018 OpenBSD-current