concatenate in the monero world

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I am trying to replicate in python the calculation of public key derivation, that is: P = H(aR||i)G +B where i is the output index in a transaction.



I got all the operators figured out (using a python ed25519 implementation) except for the concatenation.
How is concatenation (||) defined in Monero? Is it taking the two scalars as strings and calculate their hash? bitwise or? etc...



Thanks






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    I am trying to replicate in python the calculation of public key derivation, that is: P = H(aR||i)G +B where i is the output index in a transaction.



    I got all the operators figured out (using a python ed25519 implementation) except for the concatenation.
    How is concatenation (||) defined in Monero? Is it taking the two scalars as strings and calculate their hash? bitwise or? etc...



    Thanks






    cryptonote







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      I am trying to replicate in python the calculation of public key derivation, that is: P = H(aR||i)G +B where i is the output index in a transaction.



      I got all the operators figured out (using a python ed25519 implementation) except for the concatenation.
      How is concatenation (||) defined in Monero? Is it taking the two scalars as strings and calculate their hash? bitwise or? etc...



      Thanks






      cryptonote







      share|improve this question













      I am trying to replicate in python the calculation of public key derivation, that is: P = H(aR||i)G +B where i is the output index in a transaction.



      I got all the operators figured out (using a python ed25519 implementation) except for the concatenation.
      How is concatenation (||) defined in Monero? Is it taking the two scalars as strings and calculate their hash? bitwise or? etc...



      Thanks






      cryptonote




      cryptonote






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      asked 4 hours ago









      Shak

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          aR is an elliptic curve point, and has a 32 byte compressed representation. i is not a scalar. Scalars, like a would have a 32 byte representation. i however is a varint. Varints are specified in section 1.2 of this document: https://tukaani.org/xz/xz-file-format.txt



          Therefore aR||i will usually be 32+1 = 33 bytes of data. The concatenation has no padding, and is simply one byte sequence followed by another.



          Also note that although the white paper says P = H(aR||i)G + B, if you look at the Monero source code it's actually P = H(8aR||i)G + B. This is to avoid small subgroup attacks, because it forces the result of 8aR to be in the subgroup of the base point G.






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            aR is an elliptic curve point, and has a 32 byte compressed representation. i is not a scalar. Scalars, like a would have a 32 byte representation. i however is a varint. Varints are specified in section 1.2 of this document: https://tukaani.org/xz/xz-file-format.txt



            Therefore aR||i will usually be 32+1 = 33 bytes of data. The concatenation has no padding, and is simply one byte sequence followed by another.



            Also note that although the white paper says P = H(aR||i)G + B, if you look at the Monero source code it's actually P = H(8aR||i)G + B. This is to avoid small subgroup attacks, because it forces the result of 8aR to be in the subgroup of the base point G.






            share|improve this answer
























              up vote
              2
              down vote













              aR is an elliptic curve point, and has a 32 byte compressed representation. i is not a scalar. Scalars, like a would have a 32 byte representation. i however is a varint. Varints are specified in section 1.2 of this document: https://tukaani.org/xz/xz-file-format.txt



              Therefore aR||i will usually be 32+1 = 33 bytes of data. The concatenation has no padding, and is simply one byte sequence followed by another.



              Also note that although the white paper says P = H(aR||i)G + B, if you look at the Monero source code it's actually P = H(8aR||i)G + B. This is to avoid small subgroup attacks, because it forces the result of 8aR to be in the subgroup of the base point G.






              share|improve this answer






















                up vote
                2
                down vote










                up vote
                2
                down vote









                aR is an elliptic curve point, and has a 32 byte compressed representation. i is not a scalar. Scalars, like a would have a 32 byte representation. i however is a varint. Varints are specified in section 1.2 of this document: https://tukaani.org/xz/xz-file-format.txt



                Therefore aR||i will usually be 32+1 = 33 bytes of data. The concatenation has no padding, and is simply one byte sequence followed by another.



                Also note that although the white paper says P = H(aR||i)G + B, if you look at the Monero source code it's actually P = H(8aR||i)G + B. This is to avoid small subgroup attacks, because it forces the result of 8aR to be in the subgroup of the base point G.






                share|improve this answer












                aR is an elliptic curve point, and has a 32 byte compressed representation. i is not a scalar. Scalars, like a would have a 32 byte representation. i however is a varint. Varints are specified in section 1.2 of this document: https://tukaani.org/xz/xz-file-format.txt



                Therefore aR||i will usually be 32+1 = 33 bytes of data. The concatenation has no padding, and is simply one byte sequence followed by another.



                Also note that although the white paper says P = H(aR||i)G + B, if you look at the Monero source code it's actually P = H(8aR||i)G + B. This is to avoid small subgroup attacks, because it forces the result of 8aR to be in the subgroup of the base point G.







                share|improve this answer












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                answered 1 hour ago









                knaccc

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