How does a public key verify a signature? - digital-signature

Question

I am trying to get a better grapple on how public/private keys work. I understand that a sender may add a digital signature to a document using his/her private key to essentially obtain a hash of the document, but what I do not understand is how the public key can be used to verify that signature.
My understanding was that public keys encrypt, private keys decrypt... can anyone help me understand?

Answer 1

Your understanding of "public keys encrypt, private keys decrypt" is correct... for data/message ENCRYPTION. For digital signatures, it is the reverse. With a digital signature, you are trying to prove that the document signed by you came from you. To do that, you need to use something that only YOU have: your private key.
A digital signature in its simplest description is a hash (SHA1, MD5, etc.) of the data (file, message, etc.) that is subsequently encrypted with the signer's private key. Since that is something only the signer has (or should have) that is where the trust comes from. EVERYONE has (or should have) access to the signer's public key.
So, to validate a digital signature, the recipient
Calculates a hash of the same data (file, message, etc.),
Decrypts the digital signature using the sender's PUBLIC key, and
Compares the 2 hash values.
If they match, the signature is considered valid. If they don't match, it either means that a different key was used to sign it, or that the data has been altered (either intentionally or unintentionally).

Answer 2

The keys work inversely:
Public key encrypts, private key decrypts (encrypting):
openssl rsautl -encrypt -inkey public.pem -pubin -in message.txt -out message.ssl
openssl rsautl -decrypt -inkey private.pem -in message.ssl -out message.txt
Private key encrypts, public key decrypts (signing):
openssl rsautl -sign -inkey private.pem -in message.txt -out message.ssl
openssl rsautl -inkey public.pem -pubin -in message.ssl -out message.txt
Below is an example script to test this whole flow with openssl.
#!/bin/sh
# Create message to be encrypted
echo "Creating message file"
echo "---------------------"
echo "My secret message" > message.txt
echo "done\n"
# Create asymmetric keypair
echo "Creating asymmetric key pair"
echo "----------------------------"
openssl genrsa -out private.pem 1024
openssl rsa -in private.pem -out public.pem -pubout
echo "done\n"
# Encrypt with public & decrypt with private
echo "Public key encrypts and private key decrypts"
echo "--------------------------------------------"
openssl rsautl -encrypt -inkey public.pem -pubin -in message.txt -out message_enc_pub.ssl
openssl rsautl -decrypt -inkey private.pem -in message_enc_pub.ssl -out message_pub.txt
xxd message_enc_pub.ssl # Print the binary contents of the encrypted message
cat message_pub.txt # Print the decrypted message
echo "done\n"
# Encrypt with private & decrypt with public
echo "Private key encrypts and public key decrypts"
echo "--------------------------------------------"
openssl rsautl -sign -inkey private.pem -in message.txt -out message_enc_priv.ssl
openssl rsautl -inkey public.pem -pubin -in message_enc_priv.ssl -out message_priv.txt
xxd message_enc_priv.ssl
cat message_priv.txt
echo "done\n"
This script outputs the following:
Creating message file
---------------------
done
Creating asymmetric key pair
----------------------------
Generating RSA private key, 1024 bit long modulus
...........++++++
....++++++
e is 65537 (0x10001)
writing RSA key
done
Public key encrypts and private key decrypts
--------------------------------------------
00000000: 31c0 f70d 7ed2 088d 9675 801c fb9b 4f95 1...~....u....O.
00000010: c936 8cd0 0cc4 9159 33c4 9625 d752 5b77 .6.....Y3..%.R[w
00000020: 5bfc 988d 19fe d790 b633 191f 50cf 1bf7 [........3..P...
00000030: 34c0 7788 efa2 4967 848f 99e2 a442 91b9 4.w...Ig.....B..
00000040: 5fc7 6c79 40ea d0bc 6cd4 3c9a 488e 9913 _.ly#...l.<.H...
00000050: 387f f7d6 b8e6 5eba 0771 371c c4f0 8c7f 8.....^..q7.....
00000060: 8c87 39a9 0c4c 22ab 13ed c117 c718 92e6 ..9..L".........
00000070: 3d5b 8534 7187 cc2d 2f94 0743 1fcb d890 =[.4q..-/..C....
My secret message
done
Private key encrypts and public key decrypts
--------------------------------------------
00000000: 6955 cdd0 66e4 3696 76e1 a328 ac67 4ca3 iU..f.6.v..(.gL.
00000010: d6bb 5896 b6fe 68f1 55f1 437a 831c fee9 ..X...h.U.Cz....
00000020: 133a a7e9 005b 3fc5 88f7 5210 cdbb 2cba .:...[?...R...,.
00000030: 29f1 d52d 3131 a88b 78e5 333e 90cf 3531 )..-11..x.3>..51
00000040: 08c3 3df8 b76e 41f2 a84a c7fb 0c5b c3b2 ..=..nA..J...[..
00000050: 9d3b ed4a b6ad 89bc 9ebc 9154 da48 6f2d .;.J.......T.Ho-
00000060: 5d8e b686 635f b6a4 8774 a621 5558 7172 ]...c_...t.!UXqr
00000070: fbd3 0c35 df0f 6a16 aa84 f5da 5d5e 5336 ...5..j.....]^S6
My secret message
done

Answer 3

If I had to re-phrase your question from how I understand it, you are asking the following:
If public key cryptography ensures that a public key can be derived from a private key, but a private key cannot be derived from a public key, then you might wonder, how can a public key decrypt a message signed with a private key without the sender exposing the private key within the signed message to the recipient? (re-read that a few times until it makes sense)
Other answers have already explained how asymmetric cryptography means that you can either:
Encrypt with public key, decrypt with matching private key (pseudocode below)
var msg = 'secret message';
var encryptedMessage = encrypt(pub_key, msg);
var decryptedMessage = decrypt(priv_key, encryptedMessage);
print(msg == decryptedMessage == 'secret message'); // True
Encrypt with private key, decrypt with matching public key (pseudocode below)
var msg = 'secret message';
var encryptedMessage = encrypt(priv_key, msg);
var decryptedMessage = decrypt(pub_key, encryptedMessage); // HOW DOES THIS WORK???
print(msg == decryptedMessage == 'secret message'); // True
We know that both example #1 and #2 work. Example #1 makes intuitive sense, while example #2 begs the original question.
Turns out, elliptic curve cryptography (also called "elliptic curve multiplication") is the answer to the original question. Elliptic curve cryptography is the mathematical relationship that makes the following conditions possible:
A public key can be mathematically generated from a private key
A private key cannot be mathematically generated from a public key (i.e. "trapdoor function")
A private key can be verified by a public key
To most, conditions #1 and #2 make sense, but what about #3?
You have two choices here:
You can go down a rabbit-hole and spend hours upon hours learning how elliptic curve cryptography works (here is a great starting point)... OR...
You can accept the properties above--just like you accept Newton's 3 laws of motion without needing to derive them yourself.
In conclusion, a public/private keypair is created using elliptic curve cryptography, which by nature, creates a public and private key that are mathematically linked in both directions, but not mathematically derived in both directions. This is what makes it possible for you to use someone's public key to verify that they signed a specific message, without them exposing their private key to you.

Answer 4

The public key encrypts and only the private key can decrypt it, and the reverse is true. They both encrypt to different hashes but each key can decrypt the other's encryption.
There are a few different ways to verify that a message came from some expected sender. For example:
The sender sends:
The message
The hash of the message encrypted with their private key
The receiver:
Decrypts the signature (2) with the public key to obtain a message, supposedly the same message as (1) but we don't know yet. We now have two messages that we need to verify are identical. So to do this, we will encrypt them both with our public key and compare the two hashes. So we will ....
Encrypt the original message (1) with the public key to obtain a hash
Encrypt the decrypted message (3) to get a second hash and compare to (4) to verify that they are identical.
If they aren't identical it means either the message was tampered with or it was signed with some other key and not the one we thought...
Another example would be for the sender to use a common hash that the receiver might know to use as well. For example:
The sender sends:
A message
Takes a known hash of the message, then encrypts the hash with the private key
The receiver:
Decrypts (2) and gets a hash value
Hashes the message (1) with the same hash used by the sender
Compares the two hashes to make sure they match
This again ensures the message wasn't tampered with and it is from the expected sender.

Answer 5

Thought I'd provide a supplemental explanation for anyone looking for something more intuitively revealing.
A big part of this confusion arises from naming 'public keys' and 'private keys' as such because how these things actually work is directly at odds with how a 'key' is understood to be.
Take encryption for example. It could be thought of as working like so:
The parties that want to be able to read the secret messages each keep a key
hidden (i.e. a private key)
The parties that want to be able to send secret messages all have the ability to obtain an unlocked locked (i.e. a public lock)
Then sending a secret message is as easy as locking it with an unlocked lock, but unlocking it afterwards can only be done with one of the hidden keys.
This allows secret messages to be sent between parties, but from an intuitive standpoint here, 'public lock' is a more suitable name than 'public key'.
However, for sending digital signatures the roles are somewhat reversed:
The party that wants to sign messages is the only one with access to the unlocked locks (i.e. a private lock)
The parties that want to verify the signature all have the ability to obtain a key (i.e. a public key)
Then what the signer does is create two identical messages: the one that anyone can read and one to accompany it, but which they lock with one of their private locks.
Then when the receiver gets the message, they can read it, and then use the public key to unlock the locked message and compare the two messages. If the messages are the same, then they know that:
The unlocked message wasn't tampered with during travel and,
The message must have been from the person who has the matching lock to their public key.
And finally, this entire system only works if anyone who wants to validate a signer's signature has an authoritative place to go to to get the matching key to the signer's locks. Otherwise, anyone can say "Hey, here's the key to so-and-so's private lock", send you a message pretending to be them but lock it with their private lock, you perform all the above steps and believe the message must actually be from the person you thought, but you're fooled because you were mislead as to the true owner of a public key.
So long as there's a trust-worthy source for retrieving a signer's public key, you'll know who the rightful owner of a public key is, and will be able to validate their signature.

Answer 6

To your question - i was looking at the RSA implementation. And got more clarity on the way a public key is used to verify the signature using a private key. Undoubtedly, the private key is not exposed. Here is how...
Trick here is to hide the private key within a function. In this case, (p-1)*(q-1).
Consider p to be the private key and e to be the public key. p is encapsulated within another function to make it hidden.
E.g., `d = (p-1)(q-1); d * e = 1` (d is the inverse of e - public key)
Data sent = [encrypted(hash), message] = [m ^d, message];
where m is the message
Suppose
'Data sent' = y
To check for the integrity we find y^e to get m. Since m ^(d*e) = m ^1 = m.
Hope this helps! :)

Answer 7

If I encrypt some text with my private key then anyone who has my public key can decrypt it. Public key being public, anyone can have it (including thieves and cheats), so what is the point of encrypting a text with my private key (given it can be decrypted with my public key, which is available publicly)?
Well it gives us authenticity. I mean, If you were able to decrypt a message with my public key, then you can say the message was by me. But, there is a but. Suppose someone decrypted a message with my public key and got the message "Hi!", does it mean I said "Hi!". It is possible that some other private key was used to encrypt a message, and coincidentally my private key decrypt it as a meaningful text instead of gibberish.
Well this is why we also need to provide the exact message with our public key. So that, the receiver can compare it with the decrypted message.
So, we provide
Public Key
Original Message
Encrypted Message
If Original Message = Decrypt(Encrypted Message, Public Key) then the message inside was definitely by me as only I have the private key.
Bonus:
Always sending the "Original Message" is not convenient. The "Original Message" can be a 4GB ISO file. It is better to calculate a hash (A one-way hash function, also known as a message digest, is a mathematical function that takes a variable-length input string and converts it into a fixed-length binary sequence that is computationally difficult to invert—that is, generate the original string from the hash) of the original message and send it.
So, now we are sending:
Public Key
Hash of the Original Message
Encrypted Hash of the Original Message
Now If Hash(Original Message) = Decrypt(Encrypted Message, Public Key) then the message inside was definitely by me as only I have the private key.

Answer 8

I think the big issue in the misunderstanding is that when people read "Asymmetric", in their heads they think "Ok, one key encrypts and the other decrypts, hence they are asymmetrical". But if you understand that Asymmetrical actually means "IF key A encrypted data, then its "sister" key B can decrypt data. If Key B was used to encrypt data, then key A can now only decrypt." Symmetric means the same key that was used to encrypt the data can be used to decrypt the data.

Answer 9

Here is an example of public key verify a signature using Python
you need to install pycryptodome. taken from here
# pip install pycryptodome
from Crypto.PublicKey import RSA
from hashlib import sha512
# create RSA key-pair
keyPair = RSA.generate(bits=1024)
public_key = (keyPair.e, keyPair.n)
private_key = (keyPair.d, keyPair.n)
msg = b'A message for signing'
hash = int.from_bytes(sha512(msg).digest(), byteorder='big')
# RSA sign the message using private key
signature = pow(hash, private_key[0], private_key[1])
# RSA verify signature using public key
hashFromSignature = pow(signature, public_key[0], public_key[1])
print("Signature valid:", hash == hashFromSignature)