I will start with a disclaimer that I am out of my depth here. A colleague was showing me a decryption routine he wrote with pycryptodomex. He had an encrypted file, a key, and a nonce (extracted from the file). He was able to decrypt the file contents in a very straight forward way.
c = Crypto.Cipher.AES.new(key, AES.MODE_GCM, nonce)
c.decrypt(encrypted_data)
You can see a similar implementation in the pycryptodome test for GCM:
cipher = AES.new(self.key_128, AES.MODE_GCM, nonce=self.nonce_96)
pt = get_tag_random("plaintext", 16 * 100)
ct = cipher.encrypt(pt)
cipher = AES.new(self.key_128, AES.MODE_GCM, nonce=self.nonce_96)
pt2 = cipher.decrypt(ct)
Unfortunately, pycryptdomex is an additional dependency that I would need to carry around and I am looking to avoid this. I have a base installation of Anaconda, which brings with it the pyCrypto and pyCA/cryptography packages. It appears that pycryptodomex is a fork of pyCrytpo, which didn't have a stable GCM implementation to begin with. When I look at the implementation for PyCA/cryptography, it looks straight forward:
cipher = Cipher(algorithms.AES(key), modes.GCM(nonce), backend=default_backend())
d = cipher.decryptor()
But when we want to decrypt content we have to call finalize_with_tag and produce an authentication tag:
d.update(encrypted_data) + d.finalize_with_tag(tag)
Unfortunately, I don't have an authentication tag nor do I know where to find it. I can't set the value to None as there is a minimum length requirement. I'm also not sure why I need to produce an authentication tag in the first place for AES GCM decryption with PyCA/Cryptography but I do not need to produce a tag when decrypting with the pycryptodomex. I'm ultimately looking for clarity on the following:
Is it possible to implement AES/GCM decryption with the Anaconda PyCA/cryptography package if I only have access to the key, nonce, and encrypted data?
Why do I need to provide an authentication tag for decryption with one implementation and not the other?
Is pycryptodomex doing something under the hood to determine the tag?
GCM without authentication tag is equivalent to CTR mode. (except the + 1 difference in starting counter value)
Calling decrypt does not verify the tag (as far as I know). You can test this yourself by altering the ciphertext just one byte. It will decrypt just fine (to a plaintext that is off by one byte). Use decrypt_and_verify (see test_invalid_mac test).
See 2.
Apologies as I can't reply to comments. Is it possible to derive the tag from the decrypted data after decryption? This PR associated with PyCA/cryptography seems to imply the exact scenario considered here.
According to the GCM spec (section 7.2: “Algorithm for the
Authenticated Decryption Function”), the tag itself is not needed
until the ciphertext has been decrypted.
Does calling d.update(encrypted_data) decrypt data successfully and d.finalize() is only needed to verify the integrity of the data?
Related
I am trying to decrypt a message with pgcrypto that is encrypted with AES GCM 256.
Specifications of the encryption:
Encryption algorithm AES
Key [secret of listener] (64-character-long hexadecimal string in configuration)
Key length 256 bits (32 bytes)
Block mode GCM
Padding None
Initialization vector In HTTP header (X-Initialization-Vector)
Authentication tag In HTTP header (X-Authentication-Tag)
So I receive a:
body
key
iv_header
auth_tag
I tried the below
with base as (
select
'F8E2F759E528CB69375E51DB2AF9B53734E393' as body,
'000102030405060708090A0B0C0D0E0F000102030405060708090A0B0C0D0E0F' as key,
'3D575574536D450F71AC76D8' as iv_header,
'19FDD068C6F383C173D3A906F7BD1D83' as auth_tag
),
out as (
select
decrypt_iv(
convert_to(concat(decode(body,'hex'),decode(auth_tag,'hex')),'LATIN1'),
decode(key, 'hex'),
decode(iv_header, 'hex'),
'aes/pad:none'
)
from base
)
select * from out
I keep getting the error decrypt_iv error: Data not a multiple of block size while I would expect to get the encoded message {"type": "PAYMENT"}
I expect that something goes wrong in the decoding and concatenating of body and auth_tag, but can't figure out what.
Some notes on what/why I did that
I concat the auth_tag to the body as several sources describe it that way.
I use the convert_to function as it seems the only way to concatenate two bytea values
I manage to decrypt this message in other languages (i.e. Snowflake or Python)
If anyone sees what I am doing wrong or has some directions it is highly appreciated.
pgcrypto only says it supports cbc and ecb. I'm not a cryptographer, but I don't think either of those is the same thing as GCM. So I don't think you can do this with pgcrypto. I don't know why it leads to the exact error you get, but it also doesn't surprise me.
If I really needed to do this inside the database, I think I do it by writing a function using pl/python3u.
I am generating RSA signature using RSA_PKCS1_PSS_PADDING. I am setting digest algorithm as SHA256 using EVP_get_digestbyname() and EVP_DigestSignInit(). And salt length parameter as -1 using EVP_PKEY_CTX_set_rsa_pss_saltlen().
I have EVP_MD_CTX, EVP_MD and EVP_PKEY_CTX structures used for signature generation.
How can I get the name of Mask generation algorithm name used by OpenSSL by default? Is there any API provided for getting it?
Edit: OpenSSL version used: 1.1.0g.
RSASSA-PSS is in practice always used with MGF1 as the Mask Generation Function. The only variation is which Message Digest is used internally by MGF1.
Sometime that's the same Message Digest as the one used for hashing the message and building the tag in PSS, because that makes the most sense. Other times it is SHA-1 because that used to be the default MD for early RSASSA-PSS APIs, thus for the associated MGF1.
In an ideal world, some attribute (in the signature, or/and in the public key certificate used to check the signature) would tell MGF1-with-such-MD, perhaps by way of some Object IDentifier like we have to specify PSS. But crypto APIs are hell.
In order to control what Message Digest is used by MGF1, we want something on the tune of what -sigopt rsa_mgf1_md:sha256 does in the openssl dgst command.
My best guess is to set the MGF1 digest using
assert(EVP_PKEY_CTX_set_rsa_mgf1_md(ctx, EVP_sha256)>=0);
or get it using EVP_PKEY_CTX_get_rsa_mgf1_md() as documented:
The EVP_PKEY_CTX_get_rsa_mgf1_md() macro gets the MGF1 digest for ctx. If not explicitly set the signing digest is used. The padding mode must have been set to RSA_PKCS1_OAEP_PADDING or RSA_PKCS1_PSS_PADDING.
Is there any way to verify the OpenSSL signature using only {signature,hashed message} pair, skipping the original file to be presented for verification?
I need to verify the signature with only {signature,hashed message} pair remotely so using the original file is cumbersome specially when its very large.
Is there any way to verify the OpenSSL signature using only hash value and without needing the original file?
Yes, but there are strings attached.
The scheme which requires the original message to be presented to the verifying function is a Signature Scheme with Appendix (SSA). A scheme like the old PKCS #1.0 signing is an example of it.
The scheme which does not require the original message is a Signature Scheme with Recovery (PSSR). In a PSSR, the encoded message is part of the signature and masked. A scheme like the new PKCS #2.0 PSSR signing is an example of it.
There are no schemes that take just a hash, as far as I know. You have to have the {message,signature} pair. Allowing the message to be disgorged from the signing or verification can be a security violation.
OpenSSL provides both of them, as does most other security libraries, like Botan, Crypto++, NSS, etc.
Also see RSA signature on TLS on Information Security Stack Exchange.
I have been trying to verify the signature with hash value remotely so using the original file is cumbersome specially when its very large.
That's the insecure thing signature schemes want to avoid....
This is one of my first approaches to "crypto in practice". I have found myself a new aim when I stumbled upon my password (AES encrypted) on my FTP server. Since I am eager to get to know new stuff, I decided I'll give it a go and try to 'recover' my password from AES cipher. If I can make it to decipher it 'on the paper' I think I will have a good enough understanding on how it works and how to call it.
As MSDN points out:
The AesProvider provider is an AES provider that uses a session key encrypted using an RSA key container that has permissions for the IIS_IUSRS group, therefore allowing IIS worker processes to encrypt and decrypt configuration encrypted with this provider.
So, the available data:
Session key (RSA encrypted) from <configProtectedData>
AES cipher from the same file (enc:AesProvider:89v45avZx.....)
Machine RSA key (obtained using aspnet_regiis -px "SampleKeys"
keys.xml -pri) (since useMachineContainer flag is true).
If I understand correctly:
In order to decrypt the password:
I need to decrypt the RSA-Encrypted session key with Machine RSA key.
Once I have the decrypted session key I use it as AES key to decrypt the password. Am I mistaken in this thinking?
If the above is correct, I will now describe my attempts:
Using Powershell (my preferable environment):
[xml]$IISXML = [xml](cat .\IISConfigKey.xml)
[System.Xml.XmlElement]$IISElement = $IISXML.RSAKeyValue
$RSA = New-Object System.Security.Cryptography.Xml.RSAKeyValue
$RSA.LoadXml($IISElement)
## Now I have RSA-key loaded as an object from exported XML
$AESSessionKey = "LIAAAZ..1aVods=" // Total length 188
## I am importing the session key from the file
$AESProviderSessionKeyBytes = (Convert-FromBase64 $AESProviderSessionKey).ToCharArray() | % { [byte]$_ }
## 'Convert-FromBase64' is my custom function, which basically converts from Base64 to String
And this seems to be the first culprit I can't get around. Casting $RSA.Key.Decrypt($AESProviderSessionKeyBytes, $true) returns an error that the data exceeded 128 bytes. Which happened in fact, as the SessionKeyBytes is of 140-length.
As the method I am calling happily throws exceptions at me, I have no idea what to try next. The sessionKey seems too long to be RSA-encrypted? Or maybe I should divide it? Or maybe I am just mistaken in principle that it is RSA encrypted.. I tried couple of versions, but none of them progressed me any closer.
Hope you can point me in the right direction!
We're developing a service that will accept a POST request. Some of the POST data will need to be encrypted before the POST as it will be stored in hidden fields on a form.
The application is written in C#, but we want third party clients to be able to easily integrate with it. We find that most clients use PHP, Classic ASP or VB.Net.
The third parties should only be doing the encryption. We'd do the decryption. There is no two-way communication.
What are the most compatible combinations of encryption algorithm, padding mode and other options?
Assuming that you have a safe way of sharing a key (whether RSA encryption of it, retrieval over an SSH or HTTPS link, or callling the other developer on a secured phone line), any of the major modern encryptions (like AES, as mentioned by #Ed Haber) would be suitable. I would second his suggestion of AES. There should be libraries for PHP, VB, Ruby, etc.
However, remember that with "no two-way communication" you will have to find an out-of-channel method for securely getting the symmetric key to the encrypting party.
If you mean that it should be impossible for third-parties to decrypt data, then you will want to use an asymmetric encryption algorithm such as RSA. This will the third-party to encrypt data with your public key, and then only you can decrypt the data with your private key, which you do not disclose. There should be implementations of RSA available for all the languages you mentioned.
If you don't care if the third-party can decrypt the data, then AES is the way to go. You will have one key which you share with the third-parties. This key is used both for encryption and decryption.
I would use AES for the bulk data encryption and RSA for encrypting the AES Key.
If the data is small enough then just encrypt the whole thing with RSA.
Ed Haber said
I would use AES for the bulk data
encryption and RSA for encrypting the
AES Key. If the data is small enough
then just encrypt the whole thing with
RSA.
I think this is a good solution. What I would do is have your application publish an API for getting a public RSA key. When I third party wants to send you something it gets the public key. It then generates a session key to do the actual encryption using a block cipher, (ie AES), and sends the key to you by encrypting with your public key. You decrypt the session key with your private key. The third party then encrypts the data it wants to send you with AES (using a padding scheme that you also publish) and sends it to you. You decrypt it using the session key.
There are some problems with the method above. Since you are not sending any information (other than publishing your public key, you cannot control how the session key is generated. This means that third parties can use very insecure ways to of generating the session key and you will never know. A second problem is everyone who wants to send you data has to pad data for AES in the same way you do. So you will have to make sure every one co-ordinates. The second issue isn't to big, but the first could be a problem especially if you don't trust the third parties all that much to generate really good session keys from a good cryptographically secure random number generator
You could very easily implement your own XOR key-based bit encryption. With a little thought and ingenuity, you can come up with something that's more than suitable for you application.
Here's a PHP example:
function XOREncryption($InputString, $KeyPhrase){
$KeyPhraseLength = strlen($KeyPhrase);
for ($i = 0; $i < strlen($InputString); $i++){
$rPos = $i % $KeyPhraseLength;
$r = ord($InputString[$i]) ^ ord($KeyPhrase[$rPos]);
$InputString[$i] = chr($r);
}
return $InputString;
}
ColdFusion has the encrypt and decrypt functions capable of handling a range of algorithms and encodings, including the AES recommended above.
Information at: http://www.cfquickdocs.com/cf8/?getDoc=encrypt#Encrypt
Quick example code:
Key = generateSecretKey( 'AES' , 128 )
EncryptedText = encrypt( Text , Key , 'AES' , 'Hex' )
Text = decrypt( EncryptedText , Key, 'AES' , 'Hex' )
Similar functionality is available with this library for PHP:
http://www.chilkatsoft.com/p/php_aes.asp
...and Java, Python, Ruby, and others...
http://www.example-code.com/java/crypt2_aes_matchPhp.asp
http://www.example-code.com/python/aes_stringEncryption.asp
Sounds like RSA is the algorithm for you.
Why not have your server exposed over HTTPS? That way, any client which can handle HTTPS can consume the service securely.