(Tcl) RSA Encrypt/Decrypt AES Key

Demonstrates how to use RSA to protect a key for AES encryption. It can be used in this scenario: You will provide your RSA public key to any number of counterparts. Your counterpart will generate an AES key, encrypt data (or a file) using it, then encrypt the AES key using your RSA public key. Your counterpart sends you both the encrypted data and the encrypted key. Since you are the only one with access to the RSA private key, only you can decrypt the AES key. You decrypt the key, then decrypt the data using the AES key.

This example will show the entire process. (1) Generate an RSA key and save both private and public parts to PEM files. (2) Encrypt a file using a randomly generated AES encryption key. (3) RSA encrypt the AES key. (4) RSA decrypt the AES key. (5) Use it to AES decrypt the file or data.

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load ./chilkat.dll

# This example assumes the Chilkat API to have been previously unlocked.
# See Global Unlock Sample for sample code.

# ------------------
# Step 1.  Generate an RSA key and save to PEM files.

set rsa [new_CkRsa]

# Generate a 1024-bit key. 
set success [CkRsa_GenerateKey $rsa 1024]
if {$success != 1} then {
    puts [CkRsa_lastErrorText $rsa]
    delete_CkRsa $rsa
    exit
}

# Keys are exported in XML format:
set pubKeyXml [CkRsa_exportPublicKey $rsa]

set pubKey [new_CkPublicKey]

set success [CkPublicKey_LoadFromString $pubKey $pubKeyXml]
# For brevity, we are not checking the return value...
set success [CkPublicKey_SavePemFile $pubKey 0 "qa_temp/pubKey.pem"]

set privKeyXml [CkRsa_exportPrivateKey $rsa]

set privKey [new_CkPrivateKey]

set success [CkPrivateKey_LoadXml $privKey $privKeyXml]
set success [CkPrivateKey_SavePemFile $privKey "qa_temp/privKey.pem"]

# Other methods exist for saving the private key in PKCS8 format,
# both encrypted and un-encrypted..

# ------------------
# Step 2.   This is the code your counterpart will run to 
# AES encrypt a file.  It generates a random AES key
# to use for the encryption.

set crypt [new_CkCrypt2]

CkCrypt2_put_CryptAlgorithm $crypt "aes"
CkCrypt2_put_CipherMode $crypt "cbc"
CkCrypt2_put_KeyLength $crypt 256

# Generate random bytes and return the random bytes
# in a hex string:
CkCrypt2_put_EncodingMode $crypt "hex"
set randomKey [CkCrypt2_genRandomBytesENC $crypt 32]
puts "AES key = $randomKey"

# Set the key.
CkCrypt2_SetEncodedKey $crypt $randomKey "hex"

# Set the IV to a known value that will be used on both sides.
# (If desired, you could generate a random IV and protect it in the same
# way as the key...)
# The length of the IV for AES is always 16 bytes, regardless of the key size.
CkCrypt2_SetEncodedIV $crypt "000102030405060708090A0B0C0D0E0F" "hex"

# AES Encrypt the file (the file may be any size because it will
# stream the file in/out.
set success [CkCrypt2_CkEncryptFile $crypt "qa_data/hamlet.xml" "qa_temp/aesEncrypted.dat"]
if {$success != 1} then {
    puts [CkCrypt2_lastErrorText $crypt]
    delete_CkRsa $rsa
    delete_CkPublicKey $pubKey
    delete_CkPrivateKey $privKey
    delete_CkCrypt2 $crypt
    exit
}

# ------------------
# Step 3.
# ------------------
# RSA encrypt the AES key.
# We'll pretend  your counter part has the public-key PEM file and needs
# to load it.  Therefore, we'll start with a new RSA object and load
# the public key from the PEM..
set rsa2 [new_CkRsa]

set pubKey2 [new_CkPublicKey]

# For brevity, don't check the success..  
set success [CkPublicKey_LoadFromFile $pubKey2 "qa_temp/pubKey.pem"]
set pubKeyXml [CkPublicKey_getXml $pubKey2]
set success [CkRsa_ImportPublicKey $rsa2 $pubKeyXml]

# RSA encrypt the AES key and return it as a base64 encoded string.
CkRsa_put_EncodingMode $rsa2 "base64"
set bUsePrivateKey 0
set encryptedAesKey [CkRsa_encryptStringENC $rsa2 $randomKey $bUsePrivateKey]

# At this point, your counterpart sends you the encryptedAesKey,
# and the encrypted file.

# ------------------
# Step 4 - RSA decrypt the AES key.
# ------------------

# Start with a new RSA object and load the private key from the PEM.
set rsa3 [new_CkRsa]

set privKey2 [new_CkPrivateKey]

set success [CkPrivateKey_LoadPemFile $privKey2 "qa_temp/privKey.pem"]
set privKeyXml [CkPrivateKey_getXml $privKey2]
set success [CkRsa_ImportPrivateKey $rsa3 $privKeyXml]

# The encrypted AES key is encoded using base64, so set
# our EncodingMode to "base64".
CkRsa_put_EncodingMode $rsa3 "base64"

set bUsePrivateKey 1
set decryptedAesKey [CkRsa_decryptStringENC $rsa3 $encryptedAesKey $bUsePrivateKey]
puts "decryptedAesKey = $decryptedAesKey"

# ------------------
# Step 5 - AES decrypt the file.
# ------------------

set crypt2 [new_CkCrypt2]

CkCrypt2_put_CryptAlgorithm $crypt2 "aes"
CkCrypt2_put_CipherMode $crypt2 "cbc"
CkCrypt2_put_KeyLength $crypt2 256

# Set the key.
CkCrypt2_SetEncodedKey $crypt2 $decryptedAesKey "hex"

# Set the IV 
CkCrypt2_SetEncodedIV $crypt2 "000102030405060708090A0B0C0D0E0F" "hex"

# AES Decrypt the file (the file may be any size because it will
# stream the file in/out.
set success [CkCrypt2_CkDecryptFile $crypt2 "qa_temp/aesEncrypted.dat" "qa_temp/decrypted.xml"]
if {$success != 1} then {
    puts [CkCrypt2_lastErrorText $crypt2]
    delete_CkRsa $rsa
    delete_CkPublicKey $pubKey
    delete_CkPrivateKey $privKey
    delete_CkCrypt2 $crypt
    delete_CkRsa $rsa2
    delete_CkPublicKey $pubKey2
    delete_CkRsa $rsa3
    delete_CkPrivateKey $privKey2
    delete_CkCrypt2 $crypt2
    exit
}


delete_CkRsa $rsa
delete_CkPublicKey $pubKey
delete_CkPrivateKey $privKey
delete_CkCrypt2 $crypt
delete_CkRsa $rsa2
delete_CkPublicKey $pubKey2
delete_CkRsa $rsa3
delete_CkPrivateKey $privKey2
delete_CkCrypt2 $crypt2