RSA_PRIVATE_ENCRYPT(3)

📦 version 4.4.0, in alpine 3.24, arch rolling

💅 RSA_PRIVATE_ENCRYPT(3) 💅

okay bestie, this one is from the Library Functions Manual and it’s giving “low-level RSA signature operations” energy ✨

🌸 NAME 🌸

RSA_private_encrypt, RSA_public_decrypt — low level signature operations

✨ SYNOPSIS ✨

library “libcrypto” #include <openssl/rsa.h>

int
RSA_private_encrypt(int flen, const unsigned char *from,
    unsigned char *to, RSA *rsa, int padding);

int
RSA_public_decrypt(int flen, const unsigned char *from,
    unsigned char *to, RSA *rsa, int padding);

🦋 DESCRIPTION 🦋

okay so these functions handle RSA signatures at a low level — like, the bare-metal signature besties, no cap.

RSA_private_encrypt() signs the flen bytes at from (usually a message digest with an algorithm identifier) using the private key rsa and stores the signature in to. Literally to must point to RSA_size(rsa) bytes of memory. 💅

padding denotes one of the following modes:

  • RSA_PKCS1_PADDING — PKCS #1 v1.5 padding. This function does not handle the algorithmIdentifier specified in PKCS #1. When generating or verifying PKCS #1 signatures, RSA_sign(3) and RSA_verify(3) should be used.
  • RSA_NO_PADDING — Raw RSA signature. This mode should only be used to implement cryptographically sound padding modes in the application code. Signing user data directly with RSA is insecure.

RSA_public_decrypt() recovers the message digest from the flen bytes long signature at from using the signer’s public key rsa. to must point to a memory section large enough to hold the message digest (which is smaller than RSA_size(rsa) - 11). padding is the padding mode that was used to sign the data.

🎀 RETURN VALUES 🎀

RSA_private_encrypt() returns the size of the signature (i.e. RSA_size(rsa)). RSA_public_decrypt() returns the size of the recovered message digest.

On error, -1 is returned; the error codes can be obtained by ERR_get_error(3). 😭

🔥 SEE ALSO 🔥

👑 HISTORY 👑

RSA_private_encrypt() and RSA_public_decrypt() appeared in SSLeay 0.4 or earlier and have been available since OpenBSD 2.4. 🫶

RSA_NO_PADDING is available since SSLeay 0.9.0. ⭐

that’s the whole vibe, bestie: this page is from Debian, dated June 8, 2025. periodt 💖

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