Cannot parse TPM2.0 public key

I am unsuccessfully trying to use Go to parse a TPM2.0 public key. The error that I receive seems to indicate that the parsing fail due to some sort of ‘structure error’. The full error is shown below. I also have created a minimal reproducible example and posted it below. How can I resolve this parsing issue?

Error:

{"time":"2024-09-06T18:11:36.5153675Z","level":"INFO","msg":"Block type is: ","!BADKEY":"TPM2 PUBLIC KEY"}

{"time":"2024-09-06T18:11:36.517942375Z","level":"INFO","msg":"asn1: structure error: tags don't match (16 vs {class:0 tag:4 length:280 isCompound:false}) {optional:false explicit:false application:false private:false defaultValue:<nil> tag:<nil> stringType:0 timeType:0 set:false omitEmpty:false} publicKeyInfo @4"}

</code>

<code>{"time":"2024-09-06T18:11:36.5146545Z","level":"INFO","msg":"AK Public PEM is: ","!BADKEY":"-----BEGIN TPM2 PUBLIC KEY-----nBIIBGAABAAsABSRyAAAAEAAUAAsIAAAAAAABAJqe9l3iktbQ5bPENbFbNvOeg3upnNKvZZ+EcdUPltkibbs2N/DBfTLaOoGmkByHn16F0Sim8yV14cMQ75CBUvNCq/yFEnVPwJCCrM3kRo7Z+yPvftgtctKHRCKi9VouDaRfEIwIOMlYFtksyoXaS4Box29WiUn52Dm9O5AUChsgkeJB+e8DVZd2oZX4s7TGaGif1b4mYtKcTJqVzv55S05NW4TPoTcnXJbhdTjDqiNbaL5BUklyevYqj8XF4GzbmdGohF9wIYcuoNJo03ZcntScteFynReLn3BFVCZCNlvNoNN1QY6xKdKevDdwVBgfXWrOGGlSW4PpmJTH1tMeXx3xwlOM=n-----END TPM2 PUBLIC KEY-----n"} {"time":"2024-09-06T18:11:36.5153675Z","level":"INFO","msg":"Block type is: ","!BADKEY":"TPM2 PUBLIC KEY"} {"time":"2024-09-06T18:11:36.517942375Z","level":"INFO","msg":"asn1: structure error: tags don't match (16 vs {class:0 tag:4 length:280 isCompound:false}) {optional:false explicit:false application:false private:false defaultValue:<nil> tag:<nil> stringType:0 timeType:0 set:false omitEmpty:false} publicKeyInfo @4"} </code>

{"time":"2024-09-06T18:11:36.5146545Z","level":"INFO","msg":"AK Public PEM is: ","!BADKEY":"-----BEGIN TPM2 PUBLIC KEY-----nBIIBGAABAAsABSRyAAAAEAAUAAsIAAAAAAABAJqe9l3iktbQ5bPENbFbNvOeg3upnNKvZZ+EcdUPltkibbs2N/DBfTLaOoGmkByHn16F0Sim8yV14cMQ75CBUvNCq/yFEnVPwJCCrM3kRo7Z+yPvftgtctKHRCKi9VouDaRfEIwIOMlYFtksyoXaS4Box29WiUn52Dm9O5AUChsgkeJB+e8DVZd2oZX4s7TGaGif1b4mYtKcTJqVzv55S05NW4TPoTcnXJbhdTjDqiNbaL5BUklyevYqj8XF4GzbmdGohF9wIYcuoNJo03ZcntScteFynReLn3BFVCZCNlvNoNN1QY6xKdKevDdwVBgfXWrOGGlSW4PpmJTH1tMeXx3xwlOM=n-----END TPM2 PUBLIC KEY-----n"}
{"time":"2024-09-06T18:11:36.5153675Z","level":"INFO","msg":"Block type is: ","!BADKEY":"TPM2 PUBLIC KEY"}
{"time":"2024-09-06T18:11:36.517942375Z","level":"INFO","msg":"asn1: structure error: tags don't match (16 vs {class:0 tag:4 length:280 isCompound:false}) {optional:false explicit:false application:false private:false defaultValue:<nil> tag:<nil> stringType:0 timeType:0 set:false omitEmpty:false} publicKeyInfo @4"}

Example:

<code>package main

import (

"crypto/x509"

"encoding/pem"

"fmt"

"github.com/google/go-attestation/attest"

"log/slog"

"os"

)

// Global variables

var (

port string

tpmPath string

tpm *attest.TPM

logger *slog.Logger

knownEKs map[string][]byte

currentChallenge []byte

)

// TPM2PublicKey represents the structure of a TPM2 public key

type TPM2PublicKey struct {

Type uint16

NameAlg uint16

Attributes uint32

AuthPolicy []byte

Parameters TPM2PublicKeyParameters

Unique TPM2PublicKeyUnique

}

type TPM2PublicKeyParameters struct {

SymmetricAlgorithm uint16

Scheme uint16

KeyBits uint16

Exponent uint32

}

type TPM2PublicKeyUnique struct {

RSA []byte

}

func main() {

// Initialize structured logger

logger = slog.New(slog.NewJSONHandler(os.Stdout, &slog.HandlerOptions{Level: slog.LevelInfo}))

akPubPEM := "-----BEGIN TPM2 PUBLIC KEY-----nBIIBGAABAAsABSRyAAAAEAAUAAsIAAAAAAABAJqe9l3iktbQ5bPENbFbNvOeg3upnNKvZZ+EcdUPltkibbs2N/DBfTLaOoGmkByHn16F0Sim8yV14cMQ75CBUvNCq/yFEnVPwJCCrM3kRo7Z+yPvftgtctKHRCKi9VouDaRfEIwIOMlYFtksyoXaS4Box29WiUn52Dm9O5AUChsgkeJB+e8DVZd2oZX4s7TGaGif1b4mYtKcTJqVzv55S05NW4TPoTcnXJbhdTjDqiNbaL5BUklyevYqj8XF4GzbmdGohF9wIYcuoNJo03ZcntScteFynReLn3BFVCZCNlvNoNN1QY6xKdKevDdwVBgfXWrOGGlSW4PpmJTH1tMeXx3xwlOM=n-----END TPM2 PUBLIC KEY-----n"

logger.Info("AK Public PEM is: ", akPubPEM)

block, _ := pem.Decode([]byte(akPubPEM))

logger.Info("Block type is: ", block.Type)

if block == nil || block.Type != "TPM2 PUBLIC KEY" {

logger.Info("failed to decode PEM block containing public key")

os.Exit(1)

}

pub, err := x509.ParsePKIXPublicKey(block.Bytes)

if err != nil {

logger.Info(err.Error())

os.Exit(1)

}

fmt.Printf("Parsed public key: %vn", pub)

// Unmarshal ASN.1 structure

// var tpm2Pub TPM2PublicKey

// _, err := asn1.Unmarshal(block.Bytes, &tpm2Pub)

// if err != nil {

// logger.Info("ERROR: Failed to unmarshal: ", err)

// os.Exit(1)

// }

// // Convert TPM2 public key to RSA public key

// if tpm2.Algorithm(tpm2Pub.Type) != tpm2.AlgRSA {

// os.Exit(1)

// }

os.Exit(0)

}

</code>

<code>package main import ( "crypto/x509" "encoding/pem" "fmt" "github.com/google/go-attestation/attest" "log/slog" "os" ) // Global variables var ( port string tpmPath string tpm *attest.TPM logger *slog.Logger knownEKs map[string][]byte currentChallenge []byte ) // TPM2PublicKey represents the structure of a TPM2 public key type TPM2PublicKey struct { Type uint16 NameAlg uint16 Attributes uint32 AuthPolicy []byte Parameters TPM2PublicKeyParameters Unique TPM2PublicKeyUnique } type TPM2PublicKeyParameters struct { SymmetricAlgorithm uint16 Scheme uint16 KeyBits uint16 Exponent uint32 } type TPM2PublicKeyUnique struct { RSA []byte } func main() { // Initialize structured logger logger = slog.New(slog.NewJSONHandler(os.Stdout, &slog.HandlerOptions{Level: slog.LevelInfo})) akPubPEM := "-----BEGIN TPM2 PUBLIC KEY-----nBIIBGAABAAsABSRyAAAAEAAUAAsIAAAAAAABAJqe9l3iktbQ5bPENbFbNvOeg3upnNKvZZ+EcdUPltkibbs2N/DBfTLaOoGmkByHn16F0Sim8yV14cMQ75CBUvNCq/yFEnVPwJCCrM3kRo7Z+yPvftgtctKHRCKi9VouDaRfEIwIOMlYFtksyoXaS4Box29WiUn52Dm9O5AUChsgkeJB+e8DVZd2oZX4s7TGaGif1b4mYtKcTJqVzv55S05NW4TPoTcnXJbhdTjDqiNbaL5BUklyevYqj8XF4GzbmdGohF9wIYcuoNJo03ZcntScteFynReLn3BFVCZCNlvNoNN1QY6xKdKevDdwVBgfXWrOGGlSW4PpmJTH1tMeXx3xwlOM=n-----END TPM2 PUBLIC KEY-----n" logger.Info("AK Public PEM is: ", akPubPEM) block, _ := pem.Decode([]byte(akPubPEM)) logger.Info("Block type is: ", block.Type) if block == nil || block.Type != "TPM2 PUBLIC KEY" { logger.Info("failed to decode PEM block containing public key") os.Exit(1) } pub, err := x509.ParsePKIXPublicKey(block.Bytes) if err != nil { logger.Info(err.Error()) os.Exit(1) } fmt.Printf("Parsed public key: %vn", pub) // Unmarshal ASN.1 structure // var tpm2Pub TPM2PublicKey // _, err := asn1.Unmarshal(block.Bytes, &tpm2Pub) // if err != nil { // logger.Info("ERROR: Failed to unmarshal: ", err) // os.Exit(1) // } // // Convert TPM2 public key to RSA public key // if tpm2.Algorithm(tpm2Pub.Type) != tpm2.AlgRSA { // os.Exit(1) // } os.Exit(0) } </code>

package main

import (
    "crypto/x509"
    "encoding/pem"
    "fmt"
    "github.com/google/go-attestation/attest"
    "log/slog"
    "os"
)

// Global variables
var (
    port             string
    tpmPath          string
    tpm              *attest.TPM
    logger           *slog.Logger
    knownEKs         map[string][]byte
    currentChallenge []byte
)

// TPM2PublicKey represents the structure of a TPM2 public key
type TPM2PublicKey struct {
    Type       uint16
    NameAlg    uint16
    Attributes uint32
    AuthPolicy []byte
    Parameters TPM2PublicKeyParameters
    Unique     TPM2PublicKeyUnique
}

type TPM2PublicKeyParameters struct {
    SymmetricAlgorithm uint16
    Scheme             uint16
    KeyBits            uint16
    Exponent           uint32
}

type TPM2PublicKeyUnique struct {
    RSA []byte
}

func main() {

    // Initialize structured logger
    logger = slog.New(slog.NewJSONHandler(os.Stdout, &slog.HandlerOptions{Level: slog.LevelInfo}))
    
    akPubPEM := "-----BEGIN TPM2 PUBLIC KEY-----nBIIBGAABAAsABSRyAAAAEAAUAAsIAAAAAAABAJqe9l3iktbQ5bPENbFbNvOeg3upnNKvZZ+EcdUPltkibbs2N/DBfTLaOoGmkByHn16F0Sim8yV14cMQ75CBUvNCq/yFEnVPwJCCrM3kRo7Z+yPvftgtctKHRCKi9VouDaRfEIwIOMlYFtksyoXaS4Box29WiUn52Dm9O5AUChsgkeJB+e8DVZd2oZX4s7TGaGif1b4mYtKcTJqVzv55S05NW4TPoTcnXJbhdTjDqiNbaL5BUklyevYqj8XF4GzbmdGohF9wIYcuoNJo03ZcntScteFynReLn3BFVCZCNlvNoNN1QY6xKdKevDdwVBgfXWrOGGlSW4PpmJTH1tMeXx3xwlOM=n-----END TPM2 PUBLIC KEY-----n"

    logger.Info("AK Public PEM is: ", akPubPEM)

    block, _ := pem.Decode([]byte(akPubPEM))
    logger.Info("Block type is: ", block.Type)
    if block == nil || block.Type != "TPM2 PUBLIC KEY" {
        logger.Info("failed to decode PEM block containing public key")
        os.Exit(1)
    }

    pub, err := x509.ParsePKIXPublicKey(block.Bytes)
    if err != nil {
        logger.Info(err.Error())
        os.Exit(1)
    }

    fmt.Printf("Parsed public key: %vn", pub)

    // Unmarshal ASN.1 structure
    // var tpm2Pub TPM2PublicKey
    // _, err := asn1.Unmarshal(block.Bytes, &tpm2Pub)
    // if err != nil {
    //  logger.Info("ERROR: Failed to unmarshal: ", err)
    //  os.Exit(1)
    // }

    // // Convert TPM2 public key to RSA public key
    // if tpm2.Algorithm(tpm2Pub.Type) != tpm2.AlgRSA {
    //  os.Exit(1)
    // }

    os.Exit(0)

}

You have your key with a base64 payload:

<code>-----BEGIN TPM2 PUBLIC KEY-----

BIIBGAABAAsABSRyAAAAEAAUAAsIAAAAAAABAJqe9l3iktbQ5bPENbFbNvOeg3up

NKvZZ+EcdUPltkibbs2N/DBfTLaOoGmkByHn16F0Sim8yV14cMQ75CBUvNCq/yFE

VPwJCCrM3kRo7Z+yPvftgtctKHRCKi9VouDaRfEIwIOMlYFtksyoXaS4Box29WiU

52Dm9O5AUChsgkeJB+e8DVZd2oZX4s7TGaGif1b4mYtKcTJqVzv55S05NW4TPoTc

XJbhdTjDqiNbaL5BUklyevYqj8XF4GzbmdGohF9wIYcuoNJo03ZcntScteFynReL

3BFVCZCNlvNoNN1QY6xKdKevDdwVBgfXWrOGGlSW4PpmJTH1tMeXx3xwlOM=

-----END TPM2 PUBLIC KEY-----

</code>

<code>-----BEGIN TPM2 PUBLIC KEY----- BIIBGAABAAsABSRyAAAAEAAUAAsIAAAAAAABAJqe9l3iktbQ5bPENbFbNvOeg3up NKvZZ+EcdUPltkibbs2N/DBfTLaOoGmkByHn16F0Sim8yV14cMQ75CBUvNCq/yFE VPwJCCrM3kRo7Z+yPvftgtctKHRCKi9VouDaRfEIwIOMlYFtksyoXaS4Box29WiU 52Dm9O5AUChsgkeJB+e8DVZd2oZX4s7TGaGif1b4mYtKcTJqVzv55S05NW4TPoTc XJbhdTjDqiNbaL5BUklyevYqj8XF4GzbmdGohF9wIYcuoNJo03ZcntScteFynReL 3BFVCZCNlvNoNN1QY6xKdKevDdwVBgfXWrOGGlSW4PpmJTH1tMeXx3xwlOM= -----END TPM2 PUBLIC KEY----- </code>

-----BEGIN TPM2 PUBLIC KEY-----
BIIBGAABAAsABSRyAAAAEAAUAAsIAAAAAAABAJqe9l3iktbQ5bPENbFbNvOeg3up
NKvZZ+EcdUPltkibbs2N/DBfTLaOoGmkByHn16F0Sim8yV14cMQ75CBUvNCq/yFE
VPwJCCrM3kRo7Z+yPvftgtctKHRCKi9VouDaRfEIwIOMlYFtksyoXaS4Box29WiU
52Dm9O5AUChsgkeJB+e8DVZd2oZX4s7TGaGif1b4mYtKcTJqVzv55S05NW4TPoTc
XJbhdTjDqiNbaL5BUklyevYqj8XF4GzbmdGohF9wIYcuoNJo03ZcntScteFynReL
3BFVCZCNlvNoNN1QY6xKdKevDdwVBgfXWrOGGlSW4PpmJTH1tMeXx3xwlOM=
-----END TPM2 PUBLIC KEY-----

If you base64-decode the payload, you get the following binary blob (here as hex string):

<code>048201180001000b00052472000000100014000b08000000000001009a9e

f65de292d6d0e5b3c435b15b36f39e837ba934abd967e11c7543e5b6489b

6ecd8dfc305f4cb68ea069a40721e7d7a1744a29bcc95d7870c43be42054

bcd0aaff214454fc09082accde4468ed9fb23ef7ed82d72d2874422a2f55

a2e0da45f108c0838c95816d92cca85da4b8068c76f56894e760e6f4ee40

50286c82478907e7bc0d565dda8657e2ced319a1a27f56f8998b4a71326a

573bf9e52d39356e133e84dc5c96e17538c3aa235b68be415249727af62a

8fc5c5e06cdb99d1a8845f7021872ea0d268d3765c9ed49cb5e1729d178b

dc115509908d96f36834dd5063ac4a74a7af0ddc150607d75ab3861a5496

e0fa662531f5b4c797c77c7094e3

</code>

<code>048201180001000b00052472000000100014000b08000000000001009a9e f65de292d6d0e5b3c435b15b36f39e837ba934abd967e11c7543e5b6489b 6ecd8dfc305f4cb68ea069a40721e7d7a1744a29bcc95d7870c43be42054 bcd0aaff214454fc09082accde4468ed9fb23ef7ed82d72d2874422a2f55 a2e0da45f108c0838c95816d92cca85da4b8068c76f56894e760e6f4ee40 50286c82478907e7bc0d565dda8657e2ced319a1a27f56f8998b4a71326a 573bf9e52d39356e133e84dc5c96e17538c3aa235b68be415249727af62a 8fc5c5e06cdb99d1a8845f7021872ea0d268d3765c9ed49cb5e1729d178b dc115509908d96f36834dd5063ac4a74a7af0ddc150607d75ab3861a5496 e0fa662531f5b4c797c77c7094e3 </code>

048201180001000b00052472000000100014000b08000000000001009a9e
f65de292d6d0e5b3c435b15b36f39e837ba934abd967e11c7543e5b6489b
6ecd8dfc305f4cb68ea069a40721e7d7a1744a29bcc95d7870c43be42054
bcd0aaff214454fc09082accde4468ed9fb23ef7ed82d72d2874422a2f55
a2e0da45f108c0838c95816d92cca85da4b8068c76f56894e760e6f4ee40
50286c82478907e7bc0d565dda8657e2ced319a1a27f56f8998b4a71326a
573bf9e52d39356e133e84dc5c96e17538c3aa235b68be415249727af62a
8fc5c5e06cdb99d1a8845f7021872ea0d268d3765c9ed49cb5e1729d178b
dc115509908d96f36834dd5063ac4a74a7af0ddc150607d75ab3861a5496
e0fa662531f5b4c797c77c7094e3

This is ASN.1 DER. So we DER-decode this. The following is breaks it down. You see, this is just a DER octet string:

<code>04 # this is an ASN.1 tag for an OCTET STRING.

820118 # length field. 0x82: length is encoded in two bytes, length: 0x0118 = 280 bytes

</code>

<code>04 # this is an ASN.1 tag for an OCTET STRING. 820118 # length field. 0x82: length is encoded in two bytes, length: 0x0118 = 280 bytes <octet string payload> </code>

04     # this is an ASN.1 tag for an OCTET STRING.
820118 # length field. 0x82: length is encoded in two bytes, length: 0x0118 = 280 bytes
<octet string payload>

DER octet string payload:

<code>0001000b00052472000000100014000b08000000000001009a9e