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Missing padding for RSA algorithm

Context used in encryption or signing operation is not associated with any padding

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Description

This defect occurs when you perform RSA encryption or signature by using a context object without associating the object with a padding scheme.

For instance, you perform encryption by using a context object that was initially not associated with a specific padding.

ret = EVP_PKEY_CTX_set_rsa_padding(ctx, RSA_NO_PADDING);
...
ret = EVP_PKEY_encrypt(ctx, out, &out_len, in, in_len)

Risk

Padding schemes remove determinism from the RSA algorithm and protect RSA operations from certain kinds of attack. Padding ensures that a given message does not lead to the same ciphertext each time it is encrypted. Without padding, an attacker can launch chosen-plaintext attacks against the cryptosystem.

Fix

Before performing an RSA operation, associate the context object with a padding scheme that is compatible with the operation.

  • Encryption: Use the OAEP padding scheme.

    For instance, use the EVP_PKEY_CTX_set_rsa_padding function with the argument RSA_PKCS1_OAEP_PADDING or the RSA_padding_add_PKCS1_OAEP function.

    ret = EVP_PKEY_CTX_set_rsa_padding(ctx, RSA_PKCS1_OAEP_PADDING);
    You can also use the PKCS#1v1.5 or SSLv23 schemes. Be aware that these schemes are considered insecure.

    You can then use functions such as EVP_PKEY_encrypt / EVP_PKEY_decrypt or RSA_public_encrypt / RSA_private_decrypt on the context.

  • Signature: Use the RSA-PSS padding scheme.

    For instance, use the EVP_PKEY_CTX_set_rsa_padding function with the argument RSA_PKCS1_PSS_PADDING.

    ret = EVP_PKEY_CTX_set_rsa_padding(ctx, RSA_PKCS1_PSS_PADDING);
    You can also use the ANSI X9.31, PKCS#1v1.5, or SSLv23 schemes. Be aware that these schemes are considered insecure.

    You can then use functions such as the EVP_PKEY_sign-EVP_PKEY_verify pair or the RSA_private_encrypt-RSA_public_decrypt pair on the context.

If you perform two kinds of operation with the same context, after the first operation, reset the padding scheme in the context before the second operation.

Examples

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#include <stddef.h>
#include <openssl/rsa.h>
#include <openssl/evp.h>

#define fatal_error() exit(-1)

int ret;
unsigned char *out_buf;
size_t out_len;


int func(unsigned char *src, size_t len){
  EVP_PKEY_CTX *ctx;
  EVP_PKEY* pkey;

  /* Key generation */
  ctx = EVP_PKEY_CTX_new_id(EVP_PKEY_RSA,NULL); 
  if (ctx == NULL) fatal_error();

  ret = EVP_PKEY_keygen_init(ctx);
  if (ret <= 0) fatal_error();
  ret = EVP_PKEY_CTX_set_rsa_keygen_bits(ctx, 2048);
  if (ret <= 0) fatal_error();
  ret = EVP_PKEY_keygen(ctx, &pkey);  
  if (ret <= 0) fatal_error();

  /* Encryption */
  EVP_PKEY_CTX_free(ctx);
  ctx = EVP_PKEY_CTX_new(pkey,NULL); 
  if (ctx == NULL) fatal_error();

  ret = EVP_PKEY_encrypt_init(ctx);
  if (ret <= 0) fatal_error();
  return EVP_PKEY_encrypt(ctx, out_buf, &out_len, src, len); 
}

In this example, before encryption with EVP_PKEY_encrypt, a specific padding is not associated with the context object ctx.

Correction — Set Padding in Context Before Encryption

One possible correction is to set the OAEP padding scheme in the context.

#include <stddef.h>
#include <openssl/rsa.h>
#include <openssl/evp.h>

#define fatal_error() exit(-1)

int ret;
unsigned char *out_buf;
size_t out_len;


int func(unsigned char *src, size_t len){
  EVP_PKEY_CTX *ctx;
  EVP_PKEY* pkey;

  /* Key generation */
  ctx = EVP_PKEY_CTX_new_id(EVP_PKEY_RSA,NULL); 
  if (ctx == NULL) fatal_error();

  ret = EVP_PKEY_keygen_init(ctx);
  if (ret <= 0) fatal_error();
  ret = EVP_PKEY_CTX_set_rsa_keygen_bits(ctx, 2048);
  if (ret <= 0) fatal_error();
  ret = EVP_PKEY_keygen(ctx, &pkey);  
  if (ret <= 0) fatal_error();

  /* Encryption */
  EVP_PKEY_CTX_free(ctx);
  ctx = EVP_PKEY_CTX_new(pkey,NULL); 
  if (ctx == NULL) fatal_error();

  ret = EVP_PKEY_encrypt_init(ctx);
  ret = EVP_PKEY_CTX_set_rsa_padding(ctx, RSA_PKCS1_OAEP_PADDING); 
  if (ret <= 0) fatal_error();
  if (ret <= 0) fatal_error();
  return EVP_PKEY_encrypt(ctx, out_buf, &out_len, src, len); 
}

Result Information

Group: Cryptography
Language: C | C++
Default: Off
Command-Line Syntax: CRYPTO_RSA_NO_PADDING
Impact: Medium

Version History

Introduced in R2018a

See Also

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