A.2 Public key algorithms (PKA)
This class of algorithms is based on a pair of keys. One key, the private one, is never exchanged with any remote
system, while the other key, the public one, can be shared with any party. The relationship between both keys is
asymmetric (asymmetric cryptography):
• A message encrypted by the private key can be read by any party with the public key. This mechanism
ensures a strong authentication of the sender since the private key has never been shared. Digital
signatures are based on this mechanism.
Figure 16. Signature
DT50956V1
John Doe1
Private key A
John Doe2
Private key A
Private key A
John Doe3
Only John Doe1 can read this.
• A message encrypted by the public key can only be read by the private key owner.
Figure 17. PKA encryption
DT50987V1
John Doe1
Private key A
John Doe2
Private key A
Private key A
John Doe3
Only John Doe1 can read this.
The main use of public key algorithms is authentication. It is also used to resolve the “key sharing” issue of
symmetric cryptography. However, this comes at the cost of more complex operations, increased computation
time and bigger memory footprint.
RSA and elliptic curve cryptography (ECC) are the most common asymmetric algorithms.
Hybrid cryptography
Common secure transfer protocols (such as Bluetooth and TLS) rely on both algorithm types. This scheme is
known as hybrid cryptography:
• Asymmetric cryptography is used first, in order to solve the symmetric key-sharing problem. A session key
is exchanged by the public key owner to the private key owner.
• Transfer confidentiality is then provided by a symmetric algorithm using the session key.
AN5156
Public key algorithms (PKA)
AN5156 - Rev 8
page 46/56
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