Reversible One Way Hash in hmac Module
The Python module hmac provides a number of functions for creating and
verifying message authentication codes (MACs). However, some of the hash
algorithms supported by hmac are insecure and should not be used. These
insecure hash algorithms include MD4, MD5, RIPEMD-160 and SHA-1.
The MD4 hash algorithm is a cryptographic hash function that was designed in the late 1980s. MD4 is no longer considered secure, and MACs created with MD4 can be easily cracked by attackers.
The MD5 hash algorithm is a cryptographic hash function that was designed in the early 1990s. MD5 is no longer considered secure, and MACs created with MD5 can be easily cracked by attackers.
RIPEMD-160 is a cryptographic hash function that was designed in 1996. It is considered to be a secure hash function, but it is not as secure as SHA-256, SHA-384, or SHA-512. In 2017, a collision attack was found for RIPEMD-160. This means that it is possible to find two different messages that have the same RIPEMD-160 hash. While this does not mean that RIPEMD-160 is completely insecure, it does mean that it is not as secure as it once was.
The SHA-1 hash algorithm is also a cryptographic hash function that was designed in the early 1990s. SHA-1 is no longer considered secure, and MACs created with SHA-1 can be easily cracked by attackers.
Example
import hmac
secret_key = "This is my secret key."
hmac_obj = hmac.new(key, digestmod="md5")
message = "This is my message.".encode()
hmac_obj.update(message)
mac = hmac_obj.digest()
Remediation
The recommendation is to swap the insecure hashing method to one of the more
secure alternatives, SHA256, SHA-384, or SHA512.
import hmac
secret_key = "This is my secret key."
hmac_obj = hmac.new(key, digestmod="sha256")
message = "This is my message.".encode()
hmac_obj.update(message)
mac = hmac_obj.digest()
False Positives
In the case of a false positive the rule can be suppressed. Simply add a
trailing or preceding comment line with either the rule ID (PY006) or
rule category name (reversible_one_way_hash).
- Using rule ID
- Using category name
import hmac
secret_key = "This is my secret key."
# suppress: PY006
hmac_obj = hmac.new(key, digestmod="md5")
message = "This is my message.".encode()
hmac_obj.update(message)
mac = hmac_obj.digest()
import hmac
secret_key = "This is my secret key."
# suppress: reversible_one_way_hash
hmac_obj = hmac.new(key, digestmod="md5")
message = "This is my message.".encode()
hmac_obj.update(message)
mac = hmac_obj.digest()