Implementing-cryptography-using-python

Chapter 8 ■ Cryptographic Applications and PKI 233
#compress the data first
blob = zlib.compress(blob)
#In determining the chunk size, determine the private key length
used in bytes
#and subtract 42 bytes (when using PKCS1_OAEP). The data will be
encrypted
#in chunks
chunk_size = 470
offset = 0
end_loop = False
encrypted = bytearray()
while not end_loop:
#The chunk
chunk = blob[offset:offset + chunk_size]
add
file
#If the data chunk is less than the chunk size, then we need to
#padding with " ". This indicates that we reached the end of the
#so we end loop here
if len(chunk) % chunk_size != 0:
end_loop = True
#chunk += b" " * (chunk_size - len(chunk))
chunk += bytes(chunk_size - len(chunk))
#Append the encrypted chunk to the overall encrypted file
encrypted += rsa_key.encrypt(chunk)
#Increase the offset by chunk size
offset += chunk_size
#Base 64 encode the encrypted file
return base64.b64encode(encrypted)
# RSA Decryption Function
def decrypt_blob(encrypted_blob, private_key):
# Import the private key and use for decryption using PKCS1_OAEP
rsakey = RSA.importKey(private_key)
rsakey = PKCS1_OAEP.new(rsakey)
# Base 64 decode the data
encrypted_blob = base64.b64decode(encrypted_blob)
# In determining the chunk size, determine the private key length
used in bytes.
# The data will be decrypted in chunks
chunk_size = 512
offset = 0
decrypted = bytearray()